Sterculia oblonga is a tropical timber tree which is also employed in herbal medicine. Different natural products have been reported in Sterculia species. For example, Sterculia foetida contains lutedin, quercetin, anthocyanins, coumarins, and hydroxybenzoic acid [1],[2]. There is scanty report on the chemical constituents of S. oblonga. The oven dried wood contains cellulose, 36.2–46.3%; furfural, 11.5–12.4%; pentosans, 19.8–21.2%; lignin, 18.2–22.7%, and ash, 1.3–3.2% [3]. It also contains tannins, 1240 mg/100 g; alkaloids, 10%; flavonoids, 3.4%; oxalate, 1.31 g/100 g; phenols, 620 mg/g; cyanide, 1.66 mg/100 g, and lipid, 4.8% [4].

Sterculia oblonga is used traditionally in parts of Cameroon and Southern Nigeria to control male infertility but there is no report on its biological activity. Increase in serum testosterone level on drug administration is by enhancing the production of luteinizing hormone, enhancing the viability of Leydig cells, reducing testicular oxidative injury, enhancing StAR gene [5]. The present work is therefore aimed at determination of the chemical constituents and effect of S. oblonga leaf essential oil on the level of serum testosterone in male white albino rats.

Fresh S. oblonga leaves were harvested from Ikot Omini, Calabar, Cross River State, Nigeria. It was authenticated by Frank Adeoye of the Herbarium unit, Botany Department, University of Calabar while white albino rats were procured from the animal house of the Physiology Department, University of Calabar.

The fresh leaves of S. oblonga (800 g) were rinsed with distilled water, crushed and immediately steam distilled for three hours. The steam distillate was collected in a separatory funnel. The lower aqueous layer was run off while the upper essential oil layer was collected.

The constituents of the essential oil were separated by gas chromatography while the individual constituents were identified by mass spectrometric analysis [6]. The compound identification was done by comparison of the obtained mass spectra with those of the standard mass spectra of organic compounds from National Institute of Standard and Technology (NIST) library [6].

Nine white albino rats each weighing 150 g were selected from the animal house. Another set weighing 160 g each were also selected for the work. The essential oil was administered orally at a dose of 1.6 g/kg of body weight for the 150 g rats and a dose of 3.0 g/kg body weight for the 160 g rats. The oil was administered twice daily for a period of one week after which they were sacrificed to obtain their blood. The blood was allowed to clot for the serum sample to be obtained. The specimens were centrifuged to separate the serum from the blood cells.

Six microplate wells for the essential oil and seventh one for control were used. Appropriate serum reference w (10 mL) was pipetted and assigned into the wells. 50 mL of the working testosterone enzyme reagent was added. The microplates were swirled gently for 30 seconds to mix. They were covered and incubated for 60 minutes at room temperature. The contents of the microplates were discarded by decantation and blotted dry with absorbent paper. A 350 mL of washing buffer was added and decanted. This was repeated two times to make a total of three washes. Manual plate washer was employed. A 100 mL of working substrate solution was added. All the reagents were added in the same order to all the wells to minimize reaction time between different wells. The mixture was incubated for 15 minutes at room temperature. Finally, 50 mL of stop solution was added to each well and gently mixed for 20 seconds. The absorbance in each well was read at 450 nm within 30 minutes of addition of the stop solution. A calibration graph of absorbance versus concentration was used for determination of testosterone level.

Table 1 shows that S. oblonga leaf essential oil has 33 constituents, 25 of which are oxygenated. It also shows the relative molecular mass, formula, and structure of these compounds. Table 2 shows that the essential oil caused rise in testosterone level in the albino rats’ blood serum and no death occurred after seven days of oral administration of the essential oil. It also shows that increase in the dose of the oil administered resulted in further increase in the serum level of testosterone.

The constituents of the essential oil include dodecanoic acid (18.99%), 2-sec-butylcyclohexanone (4.76%), 2Z-decenol (4.70%), 2,4-decadienal E, Z (2.58%), methyldodecanoic acid (4.29%), 2,4-decadienal, E,E-(7.6%), tetradecanoic acid (4.36%), n-hexadecanoic acid (6.58%), diisooctyl phthalate (5.76%), and 10-octadecanoic acid (7.8%). All these compounds are being identified for the first time in S. oblonga. A number of these compounds are fatty acids. Dodecanoic acid (i.e., lauric acid) is mainly used for soap production. In vivo experiments have shown that lauric acid and some other fatty acids are used for treatment of acne [7],[8].

None of the rats died after seven days of administration of the essential oil. This shows that the S. oblonga leaf essential oil is not toxic to the rats. At the daily dose of 1.6 g/kg of body weight, the level of testosterone increased from 0.57 to 0.65 ng/cm³ in control and sample respectively. At a daily dose of 3 g/kg of body weight testosterone level increased from 0.57 in the control to 0.74 ng/cm³ in the blood serum after seven days of administration. This further shows that the effect of S. oblonga leaf essential oil on testosterone level is dose dependent. Since the level of testosterone is directly related to the degree of male fertility [9],[10], the essential oil increases fertility in the male rats. This gives credence to the use of S. oblonga to correct male infertility. The use of herbs in health care delivery generally has the advantage of having less hazardous side effect and is environmentally friendly [11]. Although one may be tempted to hold the major constituents responsible for the observed biological activity, such activities are known to be modulated by the minor constituents which may have synergistic, additive, and antagonistic effects [12].

The S. oblonga leaf essential oil contains 33 natural products which are being reported for the first time in S. oblonga. These compounds are responsible for the observed blood serum testosterone level enhancing activity which is dose dependent. Since it is not toxic to the rats, it could serve as an alternative source of male anti-infertility drug.